Aromatic polyesters typified by polyethylene terephthalate and polyethylene-2,6-naphthalene dicarboxylate are widely used in films because they have excellent mechanical properties, dimensional stability and heat resistance. Especially, polyethylene-2,6-naphthalate has more excellent mechanical properties, dimensional stability and heat resistance than polyethylene terephthalate and are used in fields in which these requirements are severe, for example, a base film of a high-density magnetic recording medium. However, in recent years, the requirement for dimensional stability in the high-density magnetic recording media is becoming higher and higher, and a further improvement of the characteristics is desired.
Meanwhile, in Patent Documents 1 to 4, films obtained from polyethylene-1,2-diphenoxyethane-4,4′-dicarboxylate are proposed. According to these patent documents, films having high strength and low thermal shrinkage are obtained.
In polyethylene-1,2-diphenoxyethane-4,4′-dicarboxylate proposed in these patent documents, since its crystallization temperature is very low and close to a glass transition point, there was involved such a problem that promotion of the crystallization becomes vigorous with a lapse of stretching, so that sufficient stretching cannot be achieved. Then, in the above-mentioned patent documents, it is proposed to perform stretching stepwise for the purpose of carrying out uniform stretching.
However, in conventional apparatuses for manufacturing films of polyethylene terephthalate or polyethylene naphthalate, the above-mentioned stepwise stretching cannot be carried out, and there was involved such a problem that a special apparatus is needed.
Meanwhile, the requirement for an improvement of the recording density in magnetic recording media of recent years or the like is severe. Following this, there have been circumstances where the dimensional stability demanded in the base film cannot be achieved in not only the polyethylene terephthalate film but the polyethyoene-2,6-naphthalate film.    (Patent Document 1) JP-B-46-11600    (Patent Document 2) JP-A-49-51367    (Patent Document 3) JP-A-49-59176    (Patent Document 4) JP-A-50-8869